Optical storage goes deep: 1TB stored in three dimensions

Researchers in California report on the creation of a standard sized optical …

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When you drop an optical disk into your computer, gaming console, or player of choice, the machine reads information off the surface of the disc. The density of data is limited by the wavelength of the light used to read and write the data. Currently, available technology allows us to store around 25 GB of data on a single layer, so up to 50 GB of data can go on one side of a dual layer disc; some future formats are promising even more. A new research paper in this week's edition of Applied Optics describes a method of storing data throughout the volume of a disc, and its authors have built a demonstration system that uses a standard-size (120mm x 1.2mm) optical disc to store 1 TB of data.

Exploiting three dimensions for storage opens up a great deal more space for data. Given a 532 nm laser, then the maximum storage density on a disc surface is limited to 3.5x108 bits/cm2. If data is encoded in all three dimensions, then the data density can reach as high as 6.5x1012 bit/cm3. One of the authors started looking for ways to implement this back in 1989; he and a team of chemists at the University of California, Irvine have now crafted a standard-sized optical disc in which they encoded data within hundreds of layers, where each layer is capable of holding a DVD-worth of information.

The researchers needed to come up with a material in which data could be stored in three dimensions. The disc itself is made up of a polymethylmethacrylate (PMMA) matrix that holds a molecule referred to as a dye precursor (DP). This DP molecule is colorless and has an adsorption spectrum entirely below 400 nm. When exposed to an acid, however, these DP molecules will react and form Rhodamine 700, which is colored and fluoresces strongly. In addition to the DP molecules in the matrix, there are light sensitive photoacid generator (PAG) molecules. When exposed to a single UV photon or two visible photons, these molecules will break down to form a strong acid, which can convert the DP to a fluorescent dye.

A completed piece of blank media is comprised of DP and PAG molecules embedded in the PMMA matrix. Data is written to the disc using a 532nm laser pulse. This pulse lasts a mere 6.5 ps and imparts approximately 7 nJ of energy, enough to cause the PAG to decompose and release an acid that reacts with the DP to form a dot of Rhodamine 700 dye. After recording, a 635 nm laser can induce florescence in the dye molecules. The precise location of this florescence, including its depth, can be subsequently picked up using the same optics which were used to write the data to the disc.

As a proof of concept, the authors present confocal microscopy scans that show a series of test patterns burned into the media. Their demonstration device burned different patterns at specific depths and made separate, simple test tracks in a given layer. Even at depth, they could visualize individual areas of dye, each separated by approximately 5 microns. These results, according to the authors, illustrate that each layer holds approximately 5 GB worth of data, which means that the entire disc should be capable of holding about 1 Terabyte of information.

The authors also discuss some improvements that were made before the article was published. Using slightly different laser powers and frequencies, they report that it should be possible to encode information using only 250 pJ per bit—a 24th of the power used in their test device. They also discuss working with 405nm diodes, which they believe will enable them to record a BluRay disc-worth of data in each layer. This would allow three dimensional storage to squeeze 5 TBs into the space of a standard optical disc. Unfortunately, there is no indication that the researchers will be getting a product based on this technology on the market anytime soon.

Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm